Various chemical solutions are widely used as a starting solution or a detergent not only in production of semi-conductors but also in various chemical plants. Among the chemical solutions, there are those being high in reactivity or being corrosive. For vessels and pipes for storing or transporting such chemical solutions, usually a laminated article called a backing sheet is lined on inner walls thereof. A surface of the backing sheet contacting a chemical solution must be made of a material being excellent in chemical resistance, and usually a sheet of fluorine-containing resin being excellent in chemical resistance, particularly a sheet of PTFE is used.
A typical backing sheet which has been used so far is one made by adhering a heat resistant woven fabric of glass fiber or carbon fiber to a PTFE sheet. However since it is difficult to allow the PTFE sheet to adhere to other material, a heat-meltable tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (PFA) film is interposed between the heat resistant woven fabric and the PTFE sheet and then heating is carried out for thermo-fusing at a temperature of not less than a melting point of PTFE. At the time of heating for thermo-fusing, PTFE is also melted unavoidably, and as a result, lowering of crystallinity, namely lowering of a specific weight arises. On the other hand, the higher the cystallinity (higher specific weight) of PTFE is, the lower the permeation of a chemical solution through PTFE is. Therefore it is better to avoid the heating which causes lowering of the crystallinity of PTFE. However if the heating is stopped in a state of the crystallinity of PTFE remaining to a certain extent, a required adhesive strength cannot be obtained even if PFA is interposed.
In the conventional backing sheet, unintentionally a sheet of PTFE having a lowered crystallinity and a small specific weight was used as a starting material, or even if PTFE having a high crystallinity and a high specific weight was used as a starting material, lowering of crystallinity of the PTFE sheet by heating was obliged to be accepted.
Anyway, there was neither a backing sheet nor a laminated article of PTFE having an average specific weight of not less than 2.175 and an adhesive strength of practicable level.